Oil-based drilling muds have received considerably attention and enjoyed increasing utilization for drilling oil and gas wells because these fluids possess stable rheological and filtration properties at elevated temperatures. The oil-based fluid enhances borehole stability, corrosion control, and lubricity. The so-called relaxed filtration muds allows drilling to occur at a relatively rapid rate with these fluids (particularly in shale type formation). In addition, the introduction of low-aromatic mineral seal oils as a replacement for the more conventional diesel oils has also reduced the environmental concerns associated with accidental spills.
Wider use of oil-based fluids, however, is being reduced by certain factors which include, among other factors, the various environmental restraints placed on them. In offshore and inland waters, oil muds may be used but can not be discharged into the water regardless of the oil type used in the operation. Cuttings may be discharged provided they have been thoroughly clean of oil; however, cuttings-wash techniques are expensive; sometimes ineffective and can lead to expensive seabed cleanup operations.
Perricone, et. al. have extensively and eloquently reviewed the subject of high temperature filtration control additives (SPE Drilling Engineering, 358, October, 1986). Pertinent excerpts from this review are noted below.
It has been found that oil muds should be avoided in area prone to lost-circulation problems if the mud density must be maintained near the fracture pressure of the exposed formation. In directional holes larger than 12 in. (30 cm) in diameter in which a fast penetration rate is expected, he recommends not using the oil mud if other options exist.
The industry must have available other fluids systems if it is to drill the deep, expensive wells of the future. No one product is necessarily the answer to improved fluids. A total-systems concept consisting of several products must be integrated to meet the changing demands on the fluid during the course of drilling the well. These systems will require a series of new materials, each with a particular function, to be used when needed.
Researchers have look continuously at improving the thermal stability of water-based muds. In 1973, U.S. Pat. No. 3,764,530 was granted that covered the application of a low-molecular-weight sodium polyacrylate for stabilizing the rheology of aqueous muds at elevated temperatures. Earlier that year, Perricone and Young were granted a U.S. Pat. No. 3,730,900 covering the use of a sulfonated styrene/maleic anhydride copolymer for stabilizing the rheology of water-based muds at high temperatures. The primary advantage of the latter copolymer was its increased resistance to cement and calcium ion contaminations.
Improved mechanical equipment and greater emphasis on solids removal have contributed to better fluid systems. A reduction in the quantity of bentonite added to a system has aided in the control of high-temperature flow properties.
Filtration control of water-based system, particularly above 300.degree. F. (149.degree. C.), has received the attention of many investigators who attempted to develop new products to improve control of this property. The natural polymers commonly used as filtration-control agents are not effective at the higher bottomhole temperatures (BHT's). The starches and cellulose derivatives are thermally degraded by oxidation and hydrolysis reaction, producing lower-molecular-weight by-products that cause a substantial loss of viscosity and filtration-control effectiveness. The rate of degradation depends on the fluid, dissolved oxygen, pH of the mud system, and time of exposure to elevated temperature. It has been taught that starches not be used routinely above 225.degree. F. (107.degree. C.) nor carboxymethylcellulose-based products above 300.degree. C. (149.degree. C.).
Lignitic products are used for filtration control in high density water-based muds. Upon neutralization with caustic soda, lignite produces a water-soluble salt with some remaining caustic-insoluble residue. This sodium salt is quite sensitive to electrolytes and can form insoluble calcium carboxylate products in the presence of soluble calcium compounds. Improved resistance to electrolytes can be obtained by the use of higher mud alkalinities or sulfonation of the lignite. In 1976, a combination dispersing filtration-control agent containing a sulfonated lignite and a phenol-formaldehyde resin in U.S. Pat. No. 3,950,140. This product was reported to be superior in performance and less sensitive to calcium and other electrolytes. Although lignite is recognized as more resistant to thermal degradation of lignite does occur, producing intermediate acid compounds and ultimately aliphatic and aryl carboxylic acids.
Treatment level for such lignite products may vary from 1 to more than 20 lbm/bbl (2.85 to 57 kg/m.sup.3). Over treatment may lead to increase in viscosity and rate of gelation of the mud, which can be controlled only by dilution with water.
The importance of and need for improved filtration-control polymers is indicated by the number of U.S. Pat. Nos. (3,025,234; 2,775,557; 3,072,596; and 4,357,245) granted over the years in an effort to produce improved products for drilling muds. Derivatives of hydrolyzed polyacrylamide and acrylic acid are essentially the only polymers to be used commercially in drilling muds for filtration control. Because these polymers lack tolerance toward electrolyte contamination, they have limited application.
Various criteria have also been noted by Perricone, et. al. on the specific characteristics required in the preparation of an effective, thermally stable deflocculant. The following list details some of these desirable characteristics:
1. The polymer should be water-soluble and anionic. PA0 2. It should be thermally stable and resistant to alkaline hydrolysis. PA0 3. It should be an effective filtration-control additive in the presence of electrolytes. PA0 4. Its' molecular weight should be sufficient to control filtration but not so high as to effect the rheology of the system drastically or to be susceptible to shear degradation. PA0 5. It must be cost-effective. PA0 6. It should be easily handled and stored at the rig site. PA0 7. It should be environmentally acceptable.
Perricone et. al. have detailed the properties of two synthetic high molecular weight vinyl sulfonate copolymers and their utilization in controlling high temperature filtration properties in water-based drilling muds. These copolymers are more effective at high temperatures because they do not depolymerize in oxidative or hydrolytic conditions. These materials do not form unsoluble salts with the addition of soluble electrolytes.
It should be noted in this regard that the use of hydrophobic groups on water soluble polymers to enhance the rheological properties of water based fluids has been described. One approach to provide polyacrylamide based systems containing hydrophobic groups is described by Bock, et. al., U.S. Pat. No. 4,520,182 and 4,528,348. Water soluble acrylamide copolymers containing a small amount of oil soluble or hydrophobic alkylacrylamide groups were found to impart efficient viscosification to aqueous fluids. Landoll, U.S. Pat. No. 4,304,902, described copolymers of ethylene oxide with long chain epoxides which also required relatively large polymer concentration (approximately 1%) for thickening water and required surfactant for solubility due to irregularities in the polymerization. In a related case, U.S. Pat. No. 4,428,277, modified nonionic cellulose ether polymers are described. Although these polymers show enhanced viscosification relative to polymers containing hydrophobic groups, the viscosification efficiency was very low, requiring 2 to 3 weight percent polymer to provide an enhancement. The use of surfactants to enable solubility and, in turn, viscosification, by a water soluble polymer containing hydrophobic groups is described by Evani, U.S. Pat. No. 4,432,881. The hydrophobic groups claimed are attached to the polymer via an acrylate linkage which is known to have poor hydrolytic stability. In addition, the need for a surfactant to achieve solubility and thickening efficiency should make such a system very salt sensitive, as well as very sensitive to small changes in surfactant and polymer concentrations. Emmons, et. al., U.S. Pat. No. 4,395,524, teaches acrylamide copolymers as thickeners for aqueous systems. While these polymers possess hydrophobic groups they are prepared using alcohol containing solvent which are known chain transfer agents. The resulting polymers have rather low molecular weights and, thus, relatively high polymer concentrations are required to achieve reasonable viscosification of water based fluids.